When I had trouble sourcing Festos from the normal sources I went to Amazon. There, it is about the same price to buy 10 as to buy 2. I have extra caps and couplings now for a lifetime.
I currently have my fuel pump strapped to my field tank(2.5 gal) connected to large fuel can barbs from Dreamworks. The setup I currently use starts; large filtered clunk in can, 6mm Tygon to fuel barb mounted in can as a bulkhead fitting, to the pump, from the pump a 6mm Tygon line long enough to reach the fueling location, a 6mm Festo style coupling, then to a 4" length of 6mm Tygon connected to the vent nipple in the fuel can. Makes a closed circuit, and if the pump is accidently activated dumps the fuel back into the can.
Material list:
Home Depot 2.5 gall poly pro fuel can, fuel can bulkhead vent and through barbs from your favorite supplier , fuel pump, about four feet of 6mm Tygon, a 6mm coupling and a 6mm cap. The Tygon from the pump to vent coupling length I currently have is about 2 feet which is enough to keep the fuel can out of the way of servicing the plane.
I don't have a problem with the fill line blowing off with this setup, and removing the coupling from the vent line to attach it to the plane filler line leaves the fuel can vented so I can't forget to do that. No drips anywhere with this setup.
I also keep a rag handy, but haven't needed it since this setup.
John